This invention relates to an indexing mechanism and to apparatus incorporating the same. In various industries, for example the oil extraction industry, it is necessary to control certain pieces of equipment at a considerable distance from where the operator is positioned. In some industries this can be achieved by the use of radio-controlled apparatus, but this is not practicable with submarine or subterranean drilling exercises. The control apparatus can, in certain technical fields, be actuated by electrical signals passed through conventional electrical conductors but this is not satisfactory in the field of drilling for oil or gas, where severe conditions are encountered.
In the drilling field a drill string, formed of lengths of drill pipe joined in end-to-end relationship, is fed down the wellbore. Whilst it may be desirable to actuate certain apparatus at intermediate regions along the length of the drill string, often the most important apparatus to control is at that part of the drill string furthest from the operator, i.e. near the drill bit. Generally speaking the drill string can be regarded as a hollow duct, through which drilling fluid (also known as drilling mud) is passed under pressure. Under certain circumstances it is desirable to allow the drilling fluid being passed under pressure down the drill string to by-pass the drilling bit, by venting through lateral ports and being allowed to return up the bore hole. Thus, it is desirable to be able to control effectively the opening and closing of such ports or the access to, and shutting off of, such ports.
Furthermore, it may also be desirable to actuate a stabiliser incorporated in the drill string. Such a stabiliser may have a plurality of elements capable of being moved radially outwardly, under suitable actuation, so as to engage the internal surface of the wellbore, such elements and the sleeve with which they are associated then remaining prevented from rotation relative to the wellbore, whereas the mandrel within the sleeve, which mandrel forms part of the main duct of the drill string, is free to continue to rotate.
Moreover, in a three-dimensional steering tool where there is an articulated joint it is necessary to impart some degree of eccentricity to the drill string immediately upstream of the joint, the effect of which is that the drill bit, on the remote side of the articulated joint, is forced out of a rectilinear relationship, thereby enabling the drill string to be "steered".